1. Field of the Invention
The present invention relates to an improved disk shutter and a method of manufacturing a shutter for a protective envelope containing, for example, a 3.5 in. flexible disk.
2. Background of the Invention
A protective envelope containing a 3.5 in. flexible disk is conventionally provided with a U-shaped shutter which slides on the protective envelope. Some shutters are formed by pressing a thin stainless steel sheet, some shutters are formed by injection-molding a synthetic resin, such as polypropylene or polyacetal and some shutters are formed by punching out a blank from a synthetic resin sheet, such as a vinyl chloride sheet, and folding the blank.
However, there has been no shutter having both satisfactory sliding properties and satisfactory heat-resisting properties. A stainless steel shutter shows unsatisfactory sliding performance on a protective envelope formed of ABS resin or the like. Stainless shutters are liable to abrade the protective envelope and produce abraded powder. The flexible disk contained in the protective envelope likely to become contaminated by the abraded powder. This is a possible cause of drop-out in recording information on and reproducing recorded information from-the flexible disk.
A shutter formed by folding a synthetic resin sheet has poor heat resisting properties. If the protective envelope is stored in a high-temperature atmosphere, the opposite side walls of the shutter are liable to warp away from each other increasing the width W as shown in FIG. 2(B). If the width W of the shutter increases, large gaps are formed between the shutter and the protective envelope can become soiled with dust, which is also liable to cause drop-out. If filler, such as glass fibers, is added to the synthetic resin sheet to improve the heat-resisting property of the shutter, the sliding property of the shutter deteriorates. The protective envelope is then abraded by the shutter when the same slides on the protective envelope to produce abraded powder. If the synthetic resin forming the synthetic resin sheet is softened to improve the sliding property of the shutter, the heat-resisting property of the shutter will be deteriorated. Thus, the improvement of the sliding properties and heat-resisting properties are incompatible.
In manufacturing synthetic resin shutters, for instance, the appearance of the shutters can be improved by forming the shutters in different colors. However, the manufacturing cost of the shutters increases unless the shutters of different colors are manufactured in large lots. The appearance of shutters may be improved by patterns formed by multicolor, multilayer printing. However, it is difficult to select appropriate ink having sufficiently high durability. Metallic relief printing, in particular, is impractical.